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United States Patent |
5,157,303
|
Polaert
|
October 20, 1992
|
Cathode ray tube comprising a photodeflector
Abstract
A cathode ray tube includes an electrostatic deflection system along the
path of an electron beam e.sub.f and between an electron source and a
display screen. The deflection system includes at least an electrostatic
photodeflector including a photodetector which, in response to an incident
light radiation, creates electric charges e.sub.p which modify the
electric deflection field of the photodeflector. The photodeflector may be
made up of three electrodes or two electrodes so that the electron beam
e.sub.f and the electric charges e.sub.p generated are or are not situated
in the same space. The photodetector may be a photocathode or a
photodiode. The structure may be repetitive in order to form a distributed
photodeflector along the path of the electron beam e.sub.f. The cathode
ray tube may be used as part of an oscilloscope.
Inventors:
|
Polaert; Remy (Villecresnes, FR)
|
Assignee:
|
U.S. Philips Corporation (New York, NY)
|
Appl. No.:
|
574621 |
Filed:
|
August 28, 1990 |
Foreign Application Priority Data
Current U.S. Class: |
315/410; 250/214VT; 313/439 |
Intern'l Class: |
H01J 029/74 |
Field of Search: |
315/410,10
313/384,388,432,439
250/213 VT
|
References Cited
U.S. Patent Documents
3774236 | Nov., 1973 | Schlesinger | 313/384.
|
Primary Examiner: Blum; Theodore M.
Attorney, Agent or Firm: Kraus; Robert J.
Claims
I claim:
1. A cathode ray tube comprising:
an evacuated housing,
a source of electrons mounted within the housing near one end thereof for
generating an electron beam e.sub.f,
a display screen mounted within the housing near the other end thereof, and
electrostatic deflection means along the path of the electron beam e.sub.f
and between the electron source and the display screen, and wherein said
deflection means comprise at least an electrostatic photodeflector
including a photodetector which in response to incident light radiation
thereon creates electric charges e.sub.p which modify the electric
deflection field of the photodeflector so as to deflect the electron beam
e.sub.f along its path before it strikes the display screen.
2. A cathode ray tube as claimed in claim 1, wherein the photodeflector
comprises a first and a second outer electrode between which a central
electrode is interposed, a first space through which the electron beam
e.sub.f passes, being defined by the central electrode and the second
outer electrode, and a second space which comprises the photodetector
being defined by the central electrode and the first outer electrode.
3. A cathode ray tube as claimed in claim 2, wherein the photodetector
includes a photocathode deposited on the electrode which is the most
negative of the electrodes defining the second space, the electric charges
e.sub.p moving in a direction from the photocathode towards the positive
electrode and the electron beam e.sub.f traversing the first space in a
substantially perpendicular direction to the direction of movement of the
electric charges e.sub.p.
4. A cathode ray tube as claimed in claim 2 or 3, wherein the first outer
electrode is set up at a negative potential, the second outer electrode is
set up at a positive potential and the central electrode is set up at an
intermediate potential.
5. A cathode ray tube as claimed in claim 2 or 3, wherein the first outer
electrode is set up at a positive potential, the second outer electrode is
set up at a negative potential, and the central electrode is set up at an
intermediate potential.
6. A cathode ray tube as claimed in claim 2 or 3, wherein the central
electrode is set up at a potential higher than the potentials of the first
and the second outer electrodes.
7. A cathode ray tube as claimed in claim 2 or 3, wherein the central
electrode is set up at a potential which is lower than the potentials of
the first and the second outer electrodes.
8. A cathode ray tube as claimed in claim 1, wherein the photodeflector
comprises two electrodes set up respectively at a positive and a negative
potential, a photocathode being deposited on a face of the negative
electrode directed towards the positive electrode, the negative electrode
being set up at the negative potential GND via an impedance Z, the
electric charges e.sub.p moving from the photocathode towards the positive
electrode and the electron beam traversing the same interelectrode space
in a substantially perpendicular direction to the electric charges.
9. A cathode ray tube as claimed in claim 8, wherein the quiescent
deflection of the path of the electron beam e.sub.p is compensated for by
correction means.
10. A cathode ray tube as claimed in claim 1 or 2, wherein the
photodetector comprises a photodiode.
11. A cathode ray tube as claimed in claim 2, wherein the photodetector
comprises a photodiode including a silicon piece located between the
positive outer electrode and the central electrode, the electron beam
e.sub.f traversing the space defined by the central electrode and the
negative outer electrode.
12. A cathode ray tube as claimed in claims 2 or 3, wherein the electron
beam e.sub.f is deflected by the combination of an electric signal applied
to at least one of the electrodes with an optical signal applied to the
photodetector.
13. A cathode ray tube as claimed in claim 2 or 3, wherein at least one of
the electrodes is transparent to transmit the light radiation to the
photodetector.
14. A cathode ray tube as claimed in claim 13, wherein the transparent
electrode comprises a meshed grid.
15. A cathode ray tube as claimed in claim 1, 2 or 3 which comprises
several photodeflectors forming a distributed photodeflector along the
path of the electron beam e.sub.f, the light radiation being successively
deflected from one photocathode or one photodiode to the next reflector
means.
16. A cathode ray tube as claimed in claim 15, wherein the reflector means
comprise reflectors and the distances which separate the photocathodes or
the photodiodes from the reflectors, and the distances which separate two
consecutive central electrodes, are chosen so as to ensure a synchronized
action on the electron beam e.sub.f.
17. A cathode ray tube as claimed in claim 1, 2 or 3, which comprises a
first evacuated space which includes the photodeflector and a second
evacuated space formed integrally with the first space and which comprises
the other elements of the cathode ray tube.
18. A cathode ray tube as claimed in claim 17, wherein prior to assembly
the first evacuated space forms an independent element.
19. A cathode ray tube as claimed in claim 15 wherein at least one of said
electrodes also functions as said reflector means.
20. A cathode ray tube as claimed in claims 1, 2 or 3 wherein said
electrostatic photodeflector comprises the vertical deflection means in
said cathode ray tube whereby the electron beam is vertically deflected as
a function of said incident light radiation, and
said cathode ray tube further comprises an electrostatic horizontal
deflection means along the path of the electron beam e.sub.f and in
cascade with said electrostatic photodeflector.
Description
BACKGROUND OF THE INVENTION
This invention relates to a cathode ray tube comprising electrostatic
deflection means positioned along the path of an electron beam e.sub.f
generated by an electron source.
In a cathode ray tube it is usual to deflect the path of the electron beam
by means of an electrostatic deflection produced by plates set up at
different potentials. The cathode ray tube usually comprises a pair of
plates for the horizontal deflection on which a time base is applied and a
pair of plates for the vertical deflection on which the electric signal to
be analyzed is applied. Said electric signal is introduced into the tube
by means of connectors and cables which are connected to a signal
generator. Said signals may be generated at first in forms which are
non-electric. A conversion into an electric signal is thus necessary,
which may be inconvenient or difficult in certain situations.
Said signals may have various frequencies. For rapid or high frequency
signals it may be desired to realise, for example, an oscilloscope having
a passband which covers several hundreds of MHz. This is, however,
difficult to realise with such electrostatic deflection means. Solutions
have been suggested using wave propagation techniques.
The document entitled "Les tubes a rayons cathodique a propagation d'ondes
a tres large bande" by C. Loty, Acte Electronica vol. 10, no. 4, 1966, pp.
351-361, reveals a solution using a wave line in the form of a helix. In
this case a wave line of constant division is constituted by a coiled wire
conductor along which the wave propagates at the speed of light according
to a three-dimensional structure. An oscilloscope based on such a
structure has a very high passband. However, the signals which are to be
analysed and which act on the electrostatic deflection of the electron
beam are to be introduced in an electric form by means of connection
cables which have non-negligible capacitances. In practice, there is
always the problem of sensitivity and the designer is required to
establish a compromise between the speed and the sensitivity of the
deflection of the electron beam.
So when light phenomena are analysed which may be of an extremely short
duration, a considerable part of the rapid information which they comprise
may be concealed and may even be lost by said introduction difficulties of
the electric signals in the cathode ray tube, which makes the
inconveniences even worse.
So, the solution to the problem posed is to avoid the conversion of optical
signals. Moreover, it also may be desired to preserve in the tube a great
sensitivity and a high speed for the analysis of such rapid light signals.
SUMMARY OF THE INVENTION
The solution according to the invention is that the electrostatic
deflection means comprise at least an electrostatic photodeflector
including a photodetector which, under the action of an incident light
beam, produces electric charges e.sub.p which modify the electric
deflection field of the photodeflector.
Advantageously, the light radiation is not converted into an electric
signal before its introduction into the cathode ray tube and so the
information which it comprises is better preserved. There is thus direct
intervention of the light radiation on the electron beam.
This is very useful not only in devices which are to respond rapidly to the
action of the light radiation, but also in devices which are less rapid by
taking advantage of the absence of the transformation of the light
radiation into an electric signal outside of the said device.
It is a principle of the invention to send the light radiation to be
detected directly onto one of the deflection plates via a window placed in
the side of the cathode ray tube. Said deflection plate may be covered by
a photodetector which depends on the spectral region of the light
radiation to be detected. When said photodetector receives light
radiation, a quantity of charges is created in proportion to the intensity
of the light radiation. When a positive electrode is placed in the
proximity thereof, said charges will convey and develop a positive
potential on the deflection plate. This constitutes a photodetector placed
inside of the cathode ray tube. The deflection plates and the
photodetector constitute the photodeflector. The photodetector may be a
photocathode which, under the action of an incident light radiation,
creates charges in a vacuum, or a photoelectric element, for example, a
photodiode, which, under the action of an incident light beam, creates
charges in the material of the photoelectric element. So connection
cables, connectors and bypasses between the photodetector and the
deflection plate of the cathode ray tube are omitted. A greater freedom in
the choice of the load impedance Z results.
In particular, it is no longer necessary to have an impedance adapted to
that of a connection cable (typically Z=50 .OMEGA.), and it is possible to
adopt an impedance of a high value and to augment considerably the
vertical detection sensitivity. Thus, if the impedance Z is a resistance
of 1000 Ohm accompanied by a stray capacitance of C=0.1 pf, a gain in
deflection sensitivity is obtained in the ratio 1000/50=20 for a very
short rise time (100 ps) of the photodector.
The photodeflector may comprise three electrodes comprising a first and a
second outer electrode between which is interposed a central electrode,
the central electrode defining two separated spaces, on one side a first
space where the electron beam e.sub.f passes and on the other side a
second space where the photodetector is situated.
When the photodetector is a photocathode, according to one embodiment of
the invention, the photocathode is provided on the most negative electrode
of the electrodes bounding the second space. The electric charges e.sub.p
moving from the photocathode towards the positive electrode and the
electron beam e.sub.f as it traverses the first space is deflected in a
substantially perpendicular direction.
According to other embodiments, it is possible that the central electrode
may optionally be set up at an intermediate potential, higher or lower
than the potentials of the first and second outer electrodes.
When the photodetector is a photodiode it may be constituted by one piece
of silicon placed between the positive outer electrode and the central
electrode, the electron beam e.sub.f traversing the space bounded by the
central electrode and the negative outer electrode.
Optionally it is also possible to adopt a very high charge resistance, of a
quasi-infinite value, for example 10 mOhm, to increase the detection
sensitivity. In this case the time constant becomes large with regard to
the rise time of the optical signals to be detected and at this time the
vetical deflection V.sub.y is no longer proportional to the instantaneous
light signal, but to the integral of said signal as a function of the
time:
V.sub.y =1/C.intg.i dt.
It will be obvious that the deflection sensitivity thus is inversely
proportional to the capacitance C, so proportional to the distance between
the photodetector and the positive electrode divided by the active surface
of the photodetector. Moreover, an increase of said distance
photodetector-positive electrode extends the fly time of the electrons,
that is to say the actual rise time of said interelectrode space. So there
is an optimum distance which is to be determined as a function of the
application in view.
In all of the embodiments, notably the embodiments having three electrodes,
it is advantageous to reduce the actual capacitance of the photodetector.
When the photodetector is a photocathode, one means to reduce the
capacitance between the photocathode and the deflection electrodes
consists in omitting one of the electrodes. So there exists a single
interelectrode space where the electron beam e.sub.f and the electric
charges e.sub.p are active. In this case the photodeflector has two
electrodes set up at a positive and negative potential respectively, the
photocathode being provided on the face of the negative electrode directed
towards the positive electrode. The negative electrode is set up at the
negative potential GND by an impedance Z, the electric charges e.sub.p
moving from the cathode towards the positive electrode and the electron
beam traversing the same interelectrode space in a substantially
perpendicular direction.
The light radiation must reach the photodetector to create the electric
charges e.sub.p. In accordance with the orientation of the light radiation
it may be necessary that at least one of the electrodes be transparent to
transmit the luminous radiation to the photodetector. It may be a
transparent support, for example, a metallized glass, adapted to receive
the photocathode. The electrode which faces the photocathode may also be a
meshed grid. Or, in the case of a photodiode, the silicon piece may be
covered with a transparent metal oxide.
When the photodetector has two electrodes with one space for both the
electron beam e.sub.f and the generated electric charges e.sub.p, a
quiescent permanent deflection will be produced which must normally be
compensated. Said quiescent deflection in the rest condition of the track
of the electron beam e.sub.f thus is compensated by correction means, for
example, correction coils or an electrostatic deflector.
The various embodiments which will be described relate to a photodeflector
the basic structure of which comprises three electrodes or two electrodes.
Electrode is to be understood to mean herein a plate or an element having
an appropriate shape which deflects the electron beam. The fact that the
photodetector is incorporated in the deflection means to form a
photodeflector makes it possible to increase the reaction speed to a rapid
luminous signal. However, it is also possible to increase said reaction
speed by realising a distributed photodeflector which comprises several
photodetectors arranged along the track of the electron beam e.sub.f, the
light radiation being successively deflected from one cathode or
photodiode to the next by means of reflectors. The best results are
obtained when the distances which separate the photocathodes or the
photodiodes of the reflectors on the one hand and the distances which
separate two consecutive central electrodes on the other hand are
determined so as to ensure a synchronized action on the electron beam
e.sub.f.
The photodeflector or the distributed photodeflector may be provided inside
a single space which has been evacuated and which comprises all the
elements of a cathode ray tube. However, in the case of a three-electrode
embodiment, in order to facilitate the industrial manufacture, it is
possible to insulate the space comprising the photodeflector from the
space comprising the other elements of the cathode ray tube. So, when it
concerns a photocathode it is possible to independently produce the
thermal treatments which are necessary for the formation of the
photocathode on the one hand and of the cathode of the electron beam
(electron source) on the other hand so that they are not mutually damaged
thereby. After the assembly of the CRT said two spaces may remain
noncommunicating but form one assembly mechanically after they have been
adapted in position.
BRIEF DESCRIPTION OF THE DRAWING
The invention will now be described in greater detail with reference to the
embodiments and the examples shown in the accompanying drawings, in which:
FIG. 1 shows a known cathode ray tube;
FIGS. 2A, 2B are two diagrams of a photodeflector having three electrodes
and provided with a photocathode according to the invention;
FIG. 3 is a diagram of a photodeflector having two electrodes and provided
with a photocathode according to the invention;
FIGS. 4A, 4B are diagrams of a photodeflector comprising a photodiode;
FIGS. 5A to 5D are diagrams of an embodiment of a distributed
photodeflector;
FIGS. 6A, 6B show an example of an embodiment of the distributed
photodeflector according to the perspective diagram of FIG. 5B;
FIGS. 7A, 7B show an example of an embodiment of the distributed
photodeflector according to the electric circuit diagram of FIG. 5A;
FIG. 8 shows an example of an embodiment of a cathode ray tube according to
the invention formed with two separate spaces.
DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 shows a prior-art cathode ray tube. It comprises an evacuated space
10 in which an electron gun 11 emits an electron beam e.sub.f which is
deflected (beam 14) by vertical deflection plates 12 and horizontal
deflection plates 13. The deflection plates may be formed by helical lines
according to the prior art to increase the speed of deflection of the
beam. The rapid electric signals to be analysed are introduced by
electrical connectors, not shown.
According to the invention at least one of the deflection means is replaced
by a photodeflector.
FIG. 2A shows a photodeflector having three electrodes comprising a first
outer electrode 20, a second outer electrode 21 and a central electrode
22. The electron beam e.sub.f passes in the space between the electrodes
21 and 22. The first outer electrode 20 is set up at a positive potential
HT, the second outer electrode 21 is set up at a negative potential GND,
and the central electrode 22 is set up at an intermediate potential. On
the most negative electrode of the electrodes 20 and 22, i.e., the central
electrode, a photocathode 24 is deposited on the side thereof facing the
electrode 20. The central electrode 22 is connected to the negative
potential GND by a load impedance Z. Under the influence of the light
radiation 25.sub.1, 25.sub.2, 25.sub.3 the photocathode emits electrons
(e.sub.p) which are captured by the first outer electrode 20. Under the
influence of the electric current thus created the potential of the
central electrode 22 varies and the electric deflection field between the
electrodes 21 and 22 also varies, which permits of deflecting the electron
beam e.sub.f.
FIG. 2B shows another arrangement of the elements of a photodeflector
having three electrodes. The first outer electrode 20 is set up at a
negative potential GND, the second outer electrode 21 is set up at a
positive potential HT, and the central electrode 22 is set up at an
intermediate potential, being connected to the positive potential HT via a
load impedance Z. The electron beam e.sub.f passes between the electrodes
21 and 22. The photocathode is deposited on the negative electrode 20
opposite to the central electrode 22 which is at a more positive
potential. The same mechanism as described hereinbefore is produced for
deflecting the beam.
In other embodiments the central electrode may be set up at a potential
lower or higher than the potentials of the first and second outer
electrodes, with the photocathode deposited on the more negative electrode
of the electrodes 20 and 22.
FIG. 3 shows a photodeflector having two electrodes. The electron beam
e.sub.f and the electric charges e.sub.p move in the same interelectrode
space. The photocathode 24 is deposited on the electrode 22 which is
connected to the negative potential GND via impedance Z. In this case the
direct voltage of polarization between the photocathode and the positive
electrode causes the electron beam e.sub.f to be strongly deflected at
rest. Said quiescent deflection must be compensated for by correction
means:
either by inclining a priori the electron beam before it enters the
photodeflector,
or by placing a second electrostatic deflector operating in the opposite
direction and placed either above or below the photodeflector,
or by using a magnetic deflector suitably arranged so that the track of the
beam will be formed at the desired area on the screen.
FIG. 4A shows the principal electric diagram of a photodeflector comprising
a photodiode. The photodiode 40 is connected on the one hand to a positive
potential V.sub.p (lower than the high voltage HT in the case of a
photocathode) and on the other hand to the central electrode 22 connected
to ground via an impedance Z. The electron beam e.sub.f passes between the
central electrode 22 and the second outer electrode 21 set up at a
negative potential. FIG. 4B shows a diagram of a tangible implementation
of the photodeflector of FIG. 4A. The photodiode is formed from one piece
of silicon 41 placed between the first outer electrode 20 set up at a
positive potential and the central electrode 22. In order to capture the
light radiation 25.sub.1, 25.sub.2, at least one of the electrodes must be
transparent.
FIG. 5A is an electric circuit diagram of a distributed photodeflector. It
comprises a first outer electrode 20 set up at a positive potential, a
second outer electrode 21 set up at a negative potential and a number of
central electrodes 22.sub.1 to 22.sub.6. Each of the said central
electrodes has a photocathode, for example, 24.sub.1 for the electrode
22.sub.1. Each central electrode is connected to the negative potential
GND via an impedance Z.
FIG. 5B shows the optical track followed by the light radiation 50. It
begins by impinging on the first photocathode 24.sub.1. A part of the
radiation is absorbed and generates electrons (electric charges e.sub.p)
which act on the potential of the central electrode 22.sub.1 according to
the mechanisms already described. The other part of the radiation is
deflected towards the first outer electrode 20 which reflects it in turn
towards the second photocathode, and so on. The light radiation is thus
absorbed after its action on a few photocathodes. In order to keep the
advantage of the distributed photodeflector, it is desirable to divide the
absorption of the light radiation among all the photocathodes concerned
without favoring the first ones by adapting their absorption rates.
However, in order for the actions of all the individual photodeflectors to
be in phase, it is necessary to determine the distance d separating two
consecutive individual photodeflectors to adapt the optical path, followed
by the light radiation between two consecutive photocathodes, to the
distance separating a photocathode (for example, 24.sub.1) from the first
outer electrode 20. The speed of the electrons being:
V(m/s)=(2 e.V/m).sup.1/2 =5.932.times.10.sup.5 .multidot.(V).sup.1/2
where
e is the electric charge,
m is the electron mass,
V is the applied potential.
The distances d between the photocathodes are thus determined as a function
of the applied potential.
For elongating the optical path it is possible, to use instead of the first
outer electrode 20, lateral deflectors 61, 62 such as that shown in FIGS.
6A and 6B.
In order to elongate the optical path followed by the light radiation it is
also possible to realise a distributed photodeflector as shown in FIG. 5C.
Each central electrode 22.sub.1 -22.sub.6 is set up at the negative
potential by an impedance Z (see FIG. 5A). In this case the photocathode
24 is deposited on a transparent support 53 but is separated therefrom by
the semitransparent first outer electrode 20 set up at a negative
potential. The electron beam e.sub.f passes between said central
electrodes and the second outer electrode 21 set up at a positive
potential. Thus the light radiation 50 traverses the transparent support
53 and the semitransparent electrode 20, is partially absorbed and is
reflected by the photocathode 24, again traverses the same elements and is
then reflected again by a reflector 55. The successive reflection
mechanisms are then produced in the same manner as hereinbefore. In this
case the optical path may be adapted to the distance d by the positioning
of the reflector 55.
It is also possible to modify the FIG. 5C diagram by, as shown in FIG. 5D,
to ensure that the transparent support 53 is sufficiently thick so that
the light radiation does not leave the support 53 through its face 56 in
the direction of the reflector 55, so as to have a sufficiently long
optical path. The reflection may be effected either on the reflector 55
when such a reflector is joined to the support 53, or without a reflector
55 by the face 56 itself by total reflection. The thicknesses and the
positionings of the said different elements depend on the characteristics
of speed which it is desired to give to the distributed photodeflector.
FIGS. 6A, 6B show an example of an embodiment of a photodeflector according
to the FIG. 5B diagram but with lateral reflectors 61, 62.
The light radiation 50 arrives in a direction differing considerably from
the direction of propagation of the electron beam e.sub.f. The light
radiation strikes the first photocathode 24.sub.1, which is deposited on
the first central electrode 22.sub.1, and is partially absorbed and
generates electric charges e.sub.p which are captured by the first outer
electrode 20. The other part of the light radiation is reflected by the
lateral reflector 61 which sends the radiation towards the second
photocathode. At each photocathode the radiation which is not absorbed is
thus reflected towards the following photocathode, alternatively by one
and the other lateral reflector. FIG. 6B is a plan view of the
photodeflector of FIG. 6A where the outer electrodes have been omitted to
avoid complexity of the drawing. The same elements are referred to by the
same reference numerals.
The central electrodes 22.sub.1 to 22.sub.6 shown in FIG. 5 constitute
independent conductive surfaces each connected to the negative potential
GND via an impedance Z. The electric potential of each central electrode
is thus brought under the control of the electric charges e.sub.p which
are produced by each photocathode. It is possible to realize said
plurality of conductive central electrodes in different ways. FIGS. 7A and
7B show an example of another embodiment. For this purpose an insulating
support 70 is used on which the central electrodes 22.sub.1 to 22.sub.6
are provided separately and consecutively in the direction of propagation
of the electron beam e.sub.f (not shown). Each central electrode traverses
the insulating support 70 in a manner such that it appears on the two
faces of the support. The upper face (in FIG. 7B) receives the
photocathode and the lower face serves to deflect the beam. Each
photocathode (for example, 24.sub.1) is connected via an impedance Z (for
example, 71.sub.1) to the negative potential GND. The conductive
electrodes as well as the impedances Z may be realised by conventional
thin layer technologies or thick layer technologies. The photocathodes are
deposited by conventionally used methods.
The other arrangements described with the photocathodes deposited on the
negative electrodes may use the same methods for the realisation.
FIG. 8 shows an example of an embodiment of a cathode ray tube comprising a
photodeflector having three electrodes according to the invention. The
same essential elements as already described in FIG. 1 are found again,
but one of the deflectors is in this case replaced by a photodeflector.
The cathode ray tube shown is formed by two evacuated independent spaces 10
and 80.
The space 80 is formed by an evacuated envelope. It comprises the first
outer electrode 20 and the central electrode 22.sub.a comprising the
photocathode 24. Thus the space 80 may be treated independently for all
the processes required for the formation of the photocathode and which
could otherwise cause a slight pollution of the other parts of the cathode
ray tube. The space 80 may include the window which serves to introduce
the light radiation into it.
The space 10 comprises the second outer electrode 21 as well as another
central electrode 22.sub.b which is accessible from the exterior. Thus,
during the assembly of the CRT the central electrodes 22.sub.a and
22.sub.b are electrically connected together (for example soldered) and
constitute the single central electrode 22 of the photodeflector. The
central electrode 22.sub.b of the evacuated space 10 may be placed in a
reentrant part of the evacuated space 10 in order to reduce the distance
by which it is separated from the electron beam e.sub.f, and hence reduces
the capacitances, and to facilitate the positioning of the evacuated space
80.
It will be obvious that it is not necessary to use an arrangement with two
separate spaces, but instead to place all of the elements in the evacuated
space 10. The embodiments of the photodeflector described hereinbefore may
be mounted in a cathode ray tube according to similar principles which are
well known to those skilled in the art without departing from the scope of
this invention.
Such a cathode ray tube may be used to realize an oscilloscope.
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